Solid phase extraction is a chromatographic technique of frequent use in the preparation of samples for quantitative analysis, for example, via high performance liquid chromatography (HPLC) or gas chromatography (GC) (McDonald and Bouvier, eds. Solid Phase Extraction Applications Guide and Bibliography, sixth edition, Milford, Mass.: Waters (1995)). Solid phase extraction can be used to separate a component of interest in a complex solution from potentially interfering matrix elements and to concentrate the analyte to levels amenable to detection and measurement. Thus, solid phase extraction is of use in the analysis of environmental samples, where, for example, various soluble components of soils may interfere with the analysis of trace organic materials. Solid phase extraction is also of importance in the analysis of pharmaceutical agents or metabolites in blood plasma, which requires the prior removal of plasma proteins and other matrix constituents which may interfere with the analysis.
Solid phase extraction of an aqueous solution is typically performed by passing the solution through a single-use cartridge containing a chromatographic sorbent. The most commonly used sorbents consist of porous silica particles that have been functionalized on their surface with hydrophobic octyl (C.sub.8) and octadecyl (C.sub.18) functional groups. Prior to use, such sorbents must be wetted with a water-miscible polar organic solvent to solvate the alkyl chains. This increases the contact of these chains with the aqueous phase, increasing the sorbent surface area available to solutes and, therefore, retention of solutes. Such sorbents which are not pre-wetted or have dried out display poor solute retention, and, thus, inadequate separation of solution components.
The requirement that the sorbent remain wetted during the extraction procedure complicates solid phase extractions and substantially slows sample analysis. For example, solid phase extraction cartridges, in general, have differing flow rates and must be monitored individually to prevent drying out when used on a vacuum manifold, the current state of the art for processing multiple samples. This further complicates the development of instruments for automated solid phase extraction, which often incorporate elaborate safeguards to prevent drying out of the sorbent.
Thus, there is need for a solid phase extraction method which utilizes a sorbent that does not require wetting with an organic solvent or that stays wetted even if the bulk of the wetting solvent is removed during use on a vacuum manifold. Such a method would enable more rapid sample preparation for quantitative analysis, particularly for multiple samples, and allow the development of less expensive and simpler methods for automated solid phase extraction.